This invention relates generally to ultrasonic transducer arrays and, more particularly, to an array having a plurality of individual, acoustically isolated elements that are uniformly distributed along an axis which is straight, curvilinear, or both.
Ultrasonic transducer arrays are well-known in the art and have many applications, including diagnostic medical imaging, fluid flow sensing and the nondestructive testing of materials. Such applications typically require high sensitivity and broad band frequency response for optimum resolving power.
An ultrasonic transducer array typically includes a plurality of individual transducer elements that are uniformly spaced along an array axis that is straight (i.e., a linear array), or curvilinear (e.g., a concave or convex array). The transducer elements each include a piezoelectric layer. The transducer elements also include one or more overlaying acoustic matching layers, typically each one-quarter wavelength thick. The array is electrically driven by variation of the transmit timing between adjacent transducer elements to produce a focused sound beam in an imaging plane. Increased transducer performance is achieved by electrically matching the individual transducer elements to a pulser/receiver circuit, by acoustically matching the individual transducer elements to the body to be tested, and by acoustically isolating the individual elements from each other. The acoustic matching layers are commonly employed to improve the transfer of sound energy from the piezoelectric elements into the body to be tested.
In addition to electronic focusing within the imaging plane, it is also necessary to provide for out-of-plane focusing. This is typically accomplished mechanically by using piezoelectric layers having concave surfaces or by using flat piezoelectric layers in conjunction with an acoustic lens.
One known transducer array that incorporates mechanical focusing is made with a plano-concave piezoelectric substrate. The cavity formed by the concave surface is filled with a polymer mixture, such as a tungsten-epoxy mixture, and then ground flat. An epoxy layer substrate or suitable quarter wave matching layer substrate is then affixed to the flat surface of the filler layer to improve transfer of acoustic energy from the device. Individual transducer elements are formed by cutting the resulting sandwiched substrates with a dicing saw. In the cutting process, the quarter wave matching layer substrate is uncut or only partially cut through so as to leave the individual transducer elements connected. The result of this construction is to provide an array that is mechanically focused while having a flat surface as its front face. After electrical connections are made to the individual transducer elements and the array formed to its desired configuration (e.g., linear, concave, convex), a backing layer is affixed to support the transducer elements and to absorb or reflect acoustic energy transmitted from the piezoelectric substrate.
One drawback of this array is that it provides an undesirable narrow band frequency response and low sensitivity. In particular, the non-uniform thickness of the filler layer inhibits the transfer of acoustic energy over a broad frequency range from the piezoelectric material into the body being scanned. Further, narrow band frequency response increases the pulse length of the transmitted acoustic wave and thus limits the array's axial resolution. Another drawback is that the contiguous acoustic matching layer gives rise to undesirable interelement crosstalk.
Another common construction technique for making transducer arrays is described in U.S. Pat. No. 4,734,963 to Ishiyama. In that technique, a flat plate of piezoelectric material is used and a flexible printed circuit board having electrode lead patterns is bonded to a portion of a back surface of the flat plate. Similarly, flat quarter wave matching layers of uniform thickness are affixed to the front of the flat piezoelectric plate. A flexible backing plate is attached to the back surface of the piezoelectric plate and captures a portion of the flexible printed circuit board attached. The individual transducer elements are formed by cutting through the flat piezoelectric plate and corresponding flat acoustic matching layers with a dicing saw through to the flexible backing plate. The flexible backing plate is then formed along an axis that is straight, concave, or convex and bonded to a backing base. A silicone elastomer lens is affixed to the front surface of the quarter wave matching layers to effect the desired mechanical focusing of the individual elements.
One disadvantage of this construction is that the sensitivity of the transducer elements is negatively affected by the inefficiency of the silicone lens. A silicone lens results in frequency dependent losses which are high in the range commonly used for imaging arrays (3.5 to 10 Mhz). Manufacturability is also negatively affected by the requirement for precise alignment of the silicone lens with respect to individual elements of the array.
A further construction technique, described in U.S. Pat. No. 5,042,492 to Dubut, uses a concave arrangement of piezoelectric elements that are affixed along their front surfaces to a continuous, deformable, acoustic transition blade. The blade includes a metallization layer to electrically connect the front surfaces of the piezoelectric elements. The rear surfaces of the piezoelectric elements are individually connected to separate lead wires. A disadvantage of this construction is that the blade metallization and the blade itself are continuous across the piezoelectric elements, adversely affecting the transducer performance. Additionally, the individual attachment of lead wires to the piezoelectric elements is time consuming and possibly damaging to the material.
In view of the above, it should be appreciated that there is still a need for an improved array of ultrasonic transducer elements, wherein each element has a piezoelectric layer that is mechanically focused without the necessity of an acoustic lens and that is affixed to one or more uniform thickness, similarly focused, quarter wave matching layers. The individual transducer elements, including the respective piezoelectric and matching layers, should also be mechanically isolated from each other along the array axis to form independent transducer elements that are formable along a linear or curvilinear path. There is a further need for an array providing reduced lateral resonance modes and a reduced bulk acoustic impedance of the piezoelectric layers. There is also a need to reduce the time necessary to connect the individual leads and/or ground wires to the transducer elements as well as to minimize the damage caused to the transducer array during the electrical interconnection operation. The present invention satisfies this need.